Superhydrophilic coatings

ABSTRACT

A superhydrophilic coating on a substrate can be antireflective and antifogging. The coating can remain antireflective and antifogging for extended periods. The coating can include oppositely charge inorganic nanoparticles, and can be substantially free of an organic polymer. The coating can be made mechanically robust by a hydrothermal calcination.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government may have certain rights in this invention pursuantto Grant Nos. DMR-0213282 awarded by the National Science Foundation.

TECHNICAL FIELD

This invention relates to superhydrophilic coatings.

BACKGROUND

Transparent surfaces become fogged when tiny water droplets condense onthe surface, where they scatter light and often render the surfacetranslucent. Fogging frequently occurs when a cold surface suddenlycomes in contact with warm, moist air. Fogging severity can ultimatelycompromise the usefulness of the transparent material. In some cases,fogging can be a dangerous condition, for example when the foggedmaterial is a vehicle windscreen or goggle lens. Current commodityanti-fog coatings often lose effectiveness after repeated cleanings overtime, and therefore require constant reapplication to ensure theireffectiveness.

SUMMARY

Stable superhydrophilic coatings can be formed from layer-by-layerassembled films including nanoparticles, polyelectrolytes, or acombination of these. The superhydrophilic coatings can be antifogging,antireflective, or both anti-fogging and anti-reflective. The coatingscan have high transparency, high anti-fog efficiency, long environmentalstability, high scratch and abrasion resistance, and high mechanicalintegrity. Preferably, a single coatings has a combination of theseproperties. The coating can be applied to a large area substrate usingindustry scale technology, leading to low fabrication cost.

The coatings can be used in any setting where the condensation of waterdroplets on a surface is undesired, particularly where the surface is atransparent surface. Examples of such settings include sport goggles,auto windshields, windows in public transit vehicles, windows in armoredcars for law enforcement and VIP protection, solar panels, andgreen-house enclosures; Sun-Wind-Dust goggles, laser safety eyeprotective spectacles, chemical/biological protective face masks,ballistic shields for explosive ordnance disposal personnel, and visionblocks for light tactical vehicles.

In one aspect, a method of treating a surface includes depositing afirst plurality of inorganic nanoparticles having a first electrostaticcharge on a substrate, depositing an oppositely charged polyelectrolyteover the first plurality of inorganic nanoparticles, and contacting thefirst plurality of inorganic nanoparticles and the oppositely chargedpolyelectrolyte with a calcination reagent at a calcination temperature.The oppositely charged polyelectrolyte can include a second plurality ofinorganic nanoparticles. The first plurality of inorganic nanoparticlescan have a different average particle size than the second plurality ofinorganic nanoparticles. The first plurality of inorganic nanoparticlescan include a plurality of silicon dioxide nanoparticles. The secondplurality of inorganic nanoparticles can include a plurality of titaniumdioxide nanoparticles.

The calcination temperature can be less than 500° C., less than 200° C.,or less than 150° C. The calcination reagent can be water. Contactingthe first plurality of inorganic nanoparticles and the oppositelycharged polyelectrolyte with a calcination reagent at a calcinationtemperature can include contacting at a calcination pressure. Thecalcination pressure can be in the range of 10 psi to 30 psi.

The method can include repeating the steps of depositing a firstplurality of inorganic nanoparticles having a first electrostatic chargeon a substrate and depositing an oppositely charged polyelectrolyte overthe first plurality of inorganic nanoparticles; thereby forming anelectrostatic multilayer. The electrostatic multilayer can besubstantially free of an organic polymer.

In another aspect, an article includes a surface treated by the methodof claim 1.

In another aspect, a superhydrophilic surface includes a plurality ofhydrothermally calcinated inorganic nanoparticles arranged on asubstrate. The surface can have a nanoindentation modulus of greaterthan 15 GPa.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a superhydrophilic coating.

FIGS. 2A-B are graphs illustrating properties of superhydrophiliccoatings.

FIGS. 3A-D are graphs illustrating the effect of pH on nanoparticleproperties.

FIGS. 4A-B illustrate anti-reflective properties of a superhydrophiliccoating.

FIGS. 5A-C illustrate properties of superhydrophilic surfaces.

FIG. 6 is a graph depicting self-cleaning behavior of a superhydrophiliccoating.

FIGS. 7A-B are atomic force micrographs of nanoparticle coatings.

FIG. 8 is a graph illustrating optical properties of coated glass.

FIGS. 9A-B are graphs illustrating optical properties of coated glass.

FIG. 10 is a graph illustrating mechanical properties ofsuperhydrophilic coatings.

FIGS. 11A-B are atomic force micrographs of nanoparticle coatings.

DETAILED DESCRIPTION

Surfaces having a nanotexture can exhibit extreme wetting properties. Ananotexture refers to surface features, such as ridges, valleys, orpores, having nanometer (i.e., typically less than 1 micrometer)dimensions. In some cases, the features will have an average or rmsdimension on the nanometer scale, even though some individual featuresmay exceed 1 micrometer in size. The nanotexture can be a 3D network ofinterconnected pores. Depending on the structure and chemicalcomposition of a surface, the surface can be hydrophilic, hydrophobic,or at the extremes, superhydrophilic or superhydrophobic. One method tocreate the desired texture is with a polyelectrolyte multilayer.Polyelectrolyte multilayers can also confer desirable optical propertiesto surfaces, such as anti-fogging, anti-reflectivity, or reflectivity ina desired range of wavelengths. See, for example, U.S. PatentApplication Publication Nos. 2003/0215626, and 2006/0029634, and U.S.patent application Ser. No. 11/268,547, each of which is incorporated byreference in its entirety.

Hydrophilic surfaces attract water; hydrophobic surfaces repel water. Ingeneral, a non-hydrophobic surface can be made hydrophobic by coatingthe surface with a hydrophobic material. The hydrophobicity of a surfacecan be measured, for example, by determining the contact angle of a dropof water on the surface. The contact angle can be a static contact angleor dynamic contact angle. A dynamic contact angle measurement caninclude determining an advancing contact angle or a receding contactangle, or both. A hydrophobic surface having a small difference betweenadvancing and receding contact angles (i.e., low contact anglehysteresis) can be desirable. Water droplets travel across a surfacehaving low contact angle hysteresis more readily than across a surfacehaving a high contact angle hysteresis.

A surface can be superhydrophilic. A superhydrophilic surface iscompletely and instantaneously wet by water, i.e., exhibiting waterdroplet advancing contact angles of less than 5 degrees within 0.5seconds or less upon contact with water. See, for example, Bico, J. etal., Europhys. Lett. 2001, 55, 214-220, which is incorporated byreference in its entirety. At the other extreme, a surface can besuperhydrophobic, i.e. exhibiting a water droplet advancing contactangles of 150° or higher. The lotus leaf is an example of asuperhydrophobic surface (See Neinhuis, C.; Barthlott, W. Ann. Bot.1997, 79, 677; and Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1, eachof which is incorporated by reference in its entirety). The lotus leafalso exhibits very low contact angle hysteresis: the receding contactangle is within 5° of the advancing contact angle (See, for example,Chen, W.; et al. Langmuir 1999, 15, 3395; and Oner, D.; McCarthy, T. J.Langmuir 2000, 16, 7777, each of which is incorporated by reference inits entirety).

Photochemically active materials such as TiO₂ can becomesuperhydrophilic after exposure to UV radiation; or, if treated withsuitable chemical modifications, visible radiation. Surface coatingsbased on TiO₂ typically lose their superhydrophilic qualities withinminutes to hours when placed in a dark environment, although muchprogress has been made towards eliminating this potential limitation.See, for example, Gu, Z. Z.; Fujishima, A.; Sato, O. AngewandteChemie-International Edition 2002, 41, (12), 2068-2070; and Wang, R.; etal., Nature 1997, 388, (6641), 431-432; each of which is incorporated byreference in its entirety.

Textured surfaces can promote superhydrophilic behavior. Earlytheoretical work by Wenzel and Cassie-Baxter and more recent studies byQuéré and coworkers suggest that it is possible to significantly enhancethe wetting of a surface with water by introducing roughness at theright length scale. See, for example, Wenzel, R. N. J. Phys. ColloidChem. 1949, 53, 1466; Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988;Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546; Bico,J.; et al., D. Europhysics Letters 2001, 55, (2), 214-220; and Bico, J.;et al. Europhysics Letters 1999, 47, (6), 743-744, each of which isincorporated by reference in its entirety. Building on this work, it hasrecently been demonstrated that both lithographically textured surfacesand microporous surfaces can be rendered superhydrophilic. See, e.g.,McHale, G.; Shirtcliffe, N.J.; Aqil, S.; Perry, C. C.; Newton, M. I.Physical Review Letters 2004, 93, (3), which is incorporated byreference in its entirety. The intriguing possibility of switchingbetween a superhydrophobic and superhydrophilic state has also beendemonstrated with some of these surface structures. See, for example,Sun, T. L.; et al. Angewandte Chemie-International Edition 2004, 43,(3), 357-360; and Gao, Y. F.; et al. Langmuir 2004, 20, (8), 3188-3194,each of which is incorporated by reference in its entirety.

Layer-by-layer processing of polyelectrolyte multilayers can be used tomake conformal thin film coatings with molecular level control over filmthickness and chemistry. Charged polyelectrolytes can be assembled in alayer-by-layer fashion. In other words, positively- andnegatively-charged polyelectrolytes can be alternately deposited on asubstrate. One method of depositing the polyelectrolytes is to contactthe substrate with an aqueous solution of polyelectrolyte at anappropriate pH. The pH can be chosen such that the polyelectrolyte ispartially or weakly charged. The multilayer can be described by thenumber of bilayers it includes, a bilayer resulting from the sequentialapplication of oppositely charged polyelectrolytes.

In general, a polyelectrolyte is a material bearing more than a singleelectrostatic charge. The polyelectrolyte can be positively ornegatively charged (i.e., polycationic or polyanionic, respectively). Insome embodiments, a polyelectrolyte can bear both positive and negativecharges (i.e., polyzwitterionic, such as a copolymer of cationic andanionic monomers). A polyelectrolyte can be an organic polymer includinga backbone and a plurality of charged functional groups attached to thebackbone. Examples of organic polymer polyelectrolytes includesulfonated polystyrene (SPS), polyacrylic acid (PAA), poly(allylaminehydrochloride), and salts thereof. The polyelectrolyte can be aninorganic material, such as an inorganic nanoparticle. Examples ofpolyelectrolyte inorganic nanoparticles include nanoparticles of SiO₂,TiO₂, and mixtures thereof. Some polyelectrolytes can become more orless charged depending on conditions, such as temperature or pH.Oppositely charge polyelectrolytes can be attracted to one another byvirtue of electrostatic forces. This effect can be used to advantage inlayer-by-layer processing.

Layer-by-layer methods can provide a new level of molecular control overthe deposition process by simply adjusting the pH of the processingsolutions. A nonporous polyelectrolyte multilayer can form porous thinfilm structures induced by a simple acidic, aqueous process. Tuning ofthis pore forming process, for example, by the manipulation of suchparameters as salt content (ionic strength), temperature, or surfactantchemistry, can lead to the creation of micropores, nanopores, or acombination thereof. A nanopore has a diameter of less than 150 nm, forexample, between 1 and 120 nm or between 10 and 100 nm. A nanopore canhave diameter of less than 100 nm. A micropore has a diameter of greaterthan 150 nm, typically greater than 200 nm. Selection of pore formingconditions can provide control over the porosity of the coating. Forexample, the coating can be a nanoporous coating, substantially free ofmicropores. Alternatively, the coating can be a microporous coatinghaving an average pore diameters of greater than 200 nm, such as 250 nm,500 nm, 1 micron, 2 microns, 5 microns, 10 microns, or larger.

The properties of weakly charged polyelectrolytes can be preciselycontrolled by changes in pH. See, for example, G. Decher, Science 1997,277, 1232; Mendelsohn et al., Langmuir 2000, 16, 5017; Fery et al.,Langmuir 2001, 17, 3779; Shiratori et al., Macromolecules 2000, 33,4213; and U.S. Patent Application Publication No. 2003-0215626, each ofwhich is incorporated by reference in its entirety. A coating of thistype can be applied to any surface amenable to the water basedlayer-by-layer (LbL) adsorption process used to construct thesepolyelectrolyte multilayers. Because the water based process can depositpolyelectrolytes wherever the aqueous solution contacts a surface, eventhe inside surfaces of objects having a complex topology can be coated.In general, a polyelectrolyte can be applied to a surface by any methodamenable to applying an aqueous solution to a surface, such asimmersion, spraying, printing (e.g., ink jet printing), or mistdeposition.

In 1966, Iler reported that multilayers of oppositely chargednanoparticles can be assembled by the sequential adsorption ofoppositely charged nanoparticles onto substrates from aqueoussuspensions (Iler, R. K. J. Colloid Interf. Sci. 1966, 21, 569-594,which is incorporated by reference in its entirety).

Surfaces with extreme wetting behavior can be fabricated from apolyelectrolyte coating. See, for example, U.S. Patent ApplicationPublication No. 2006/0029808, which is incorporated by reference in itsentirety. A polyelectrolyte can have a backbone with a plurality ofcharged functional groups attached to the backbone. A polyelectrolytecan be polycationic or polyanionic. A polycation has a backbone with aplurality of positively charged functional groups attached to thebackbone, for example poly(allylamine hydrochloride). A polyanion has abackbone with a plurality of negatively charged functional groupsattached to the backbone, such as sulfonated polystyrene (SPS) orpoly(acrylic acid), or a salt thereof. Some polyelectrolytes can losetheir charge (i.e., become electrically neutral) depending on conditionssuch as pH. Some polyelectrolytes, such as copolymers, can include bothpolycationic segments and polyanionic segments.

Multilayer thin films containing nanoparticles of SiO₂ can be preparedvia layer-by-layer assembly (see Lvov, Y.; Ariga, K.; Onda, M.;Ichinose, I.; Kunitake, T. Langmuir 1997, 13, (23), 6195-6203, which isincorporated by reference in its entirety). Other studies describemultilayer assembly of TiO₂ nanoparticles, SiO₂ sol particles and singleor double layer nanoparticle-based anti-reflection coatings. See, forexample, Zhang, X-T.; et al. Chem. Mater. 2005, 17, 696; Rouse, J. H.;Ferguson, G. S. J. Am. Chem. Soc. 2003, 125, 15529; Sennerfors, T.; etal. Langmuir 2002, 18, 6410; Bogdanvic, G.; et al. J. Colloids InterfaceScience 2002, 255, 44; Hattori, H. Adv. Mater. 2001, 13, 51; Koo, H. Y.;et al. Adv. Mater. 2004, 16, 274; and Ahn, J. S.; Hammond, P. T.;Rubner, M. F.; Lee, I. Colloids and Surfaces A: Physicochem. Eng.Aspects 2005, 259, 45, each of which is incorporated by reference in itsentirety. Incorporation of TiO₂ nanoparticles into a multilayer thinfilm can improve the stability of the superhydrophilic state induced bylight activation. See, e.g., Kommireddy, D. S.; et al. J. Nanosci.Nanotechnol. 2005, 5, 1081, which is incorporated by reference in itsentirety.

Layer-by-layer processing can be used to apply a high-efficiencyconformal antireflective coating to virtually any surface of arbitraryshape, size, or material. See, for example, U.S. Patent ApplicationPublication No. 2003/0215626, which is incorporated by reference inentirety. The process can be used to apply the antireflective coating tomore than one surface at a time and can produce coatings that aresubstantially free of pinholes and defects, which can degrade coatingperformance. The porous coating can be antireflective. The process canbe used to form antireflective and antiglare coatings on polymericsubstrates. The simple and highly versatile process can createmolecular-level engineered conformal thin films that function aslow-cost, high-performance antireflection and antiglare coatings. Theprocess can be used to produce high-performance polymeric opticalcomponents, including flat panel displays and solar cells.

Similarly, the coating can be an antifogging coating. The antifoggingcoating can prevent condensation of light-scattering water droplets on asurface. By preventing the formation of light-scattering water dropletson the surface, the coating can help maintain optical clarity of atransparent surface, e.g., a window or display screen. The coating canbe both antireflective and antifogging. A surface of a transparentobject having the antifogging coating maintains its transparency tovisible light when compared to the same object without the antifoggingcoating under conditions that cause water condensation on the surface.Advantageously, a porous material can be simultaneously antifogging andantireflective. For example, a porous material can promote infiltrationof water droplets into pores (to prevent fogging); and the pores canalso reduce the refractive index of the coating, so that it acts as anantireflective coating.

A superhydrophilic coating can be made by depositing a polyelectrolytemultilayer film on a substrate and treating the multilayer to induce aporosity transition. The porosity transition can give rise to nanoscaleporosity in the multilayer. Nanoparticles can be applied to furtheraugment the texture of the surface. The resulting surface can besuperhydrophilic.

A superhydrophilic surface can include a polyelectrolyte multilayer. Asurface can be coated with the multilayer using a layer-by-layer method.Treatment of the multilayer can induce the formation of roughness in themultilayer. The multilayer can become a high roughness multilayer. Highroughness can be micrometer scale roughness. The high roughness surfacecan have an rms roughness of 100 nm, 150 nm, 200 nm, or greater.Treatments that induce the formation of high roughness can include anacid treatment or a salt treatment (i.e., treatment with an aqueoussolution of a salt). Formation of pores in the polyelectrolytemultilayer can lead to the development of high roughness in themultilayer. Appropriate selection of conditions (e.g., pH, temperature,processing time) can promote formation of pores of different sizes. Thepores can be micropores (e.g., pores with diameters at the micrometerscale, such as greater than 200 nm, greater than 500 nm, greater than 1micrometer, or 10 micrometers or later). A microporous polyelectrolytemultilayer can be a high roughness polyelectrolyte multilayer.

A high roughness polyelectrolyte multilayer can be formed by forming thepolyelectrolyte multilayer over a high roughness surface. When thepolyelectrolyte multilayer is formed over a high roughness surface, atreatment to increase the polyelectrolyte multilayer of thepolyelectrolyte multilayer can be optional. The high roughness surfacecan include, for example: particles, such as microparticles ormicrospheres; nanoparticles or nanospheres; or an area of elevations,ridges or depressions. The micrometer scale particles can be, forexample, particles of a clay or other particulate material. Elevations,ridges or depressions can be formed, for example, by etching, depositingmicrometer scale particles, or photolithography on a suitable substrate.

A lock-in step can prevent further changes in the structure of theporous multilayer. The lock-in can be achieved by, for example, exposureof the multilayer to chemical or thermal polymerization conditions. Thepolyelectrolytes can become cross-linked and unable to undergo furthertransitions in porosity. In some cases, chemical crosslinking step caninclude treatment of a polyelectrolyte multilayer with a carbodiimidereagent. The carbodiimide can promote the formation of crosslinksbetween carboxylate and amine groups of the polyelectrolytes. A chemicalcrosslinking step can be preferred when the polyelectrolyte multilayeris formed on a substrate that is unstable at temperatures required forcrosslinking (such as, for example, when the substrate is polystyrene).The crosslinking step can be a photocrosslinking step. Thephotocrosslinking can use a sensitizer (e.g., a light-sensitive group)and exposure to light (such as UV, visible or IR light) to achievecrosslinking. Masks can be used to form a pattern of crosslinked andnon-crosslinked regions on a surface. Other methods for crosslinkingpolymer chains of the polyelectrolyte multilayer are known.

Nanoparticles can be applied to the multilayer, to provide ananometer-scale texture or roughness to the surface. The nanoparticlescan be nanospheres such as, for example, silica nanospheres, titaniananospheres, polymer nanospheres (such as polystyrene nanospheres), ormetallic nanospheres. The nanoparticles can be metallic nanoparticles,such as gold or silver nanoparticles. The nanoparticles can havediameters of, for example, between 1 and 1000 nanometers, between 10 and500 nanometers, between 20 and 100 nanometers, or between 1 and 100nanometers. The intrinsically high wettability of silica nanoparticlesand the rough and porous nature of the multilayer surface establishfavorable conditions for extreme wetting behavior.

Superhydrophilic coatings can be created from multilayers without theneed for treating the multilayer to induce a porosity transition. Forexample, the multilayer can include a polyelectrolyte and a plurality ofhydrophilic nanoparticles. By choosing appropriate assembly conditions,a 3D nanoporous network of controllable thickness can be created withthe nanoparticles. The network can be interconnected—in other words, thenanopores can form a plurality of connected voids. Rapid infiltration(nano-wicking) of water into this network can drive the superhydrophilicbehavior.

The coating can be substantially free of organic polymers. For example,the coating can include oppositely charged inorganic nanoparticles,e.g., SiO₂ nanoparticles and TiO₂ nanoparticles.

Mechanical integrity (e.g., durability and adhesion) of a coating can beimportant in practical applications. As-assembled (i.e., prior to alock-in treatment) TiO₂/SiO₂ nanoparticle-based multilayers can haveless than ideal mechanical properties. The poor adhesion and durabilityof the as-assembled multilayer films is likely due to the absence ofinterpenetrating components (i.e., charged macromolecules) that bridgethe deposited materials together within the coatings. The mechanicalproperties of the coatings can be drastically improved by calcinatingthe as-assembled multilayers at a high temperature (e.g., 550° C.) for 3hours which leads to the fusing of the nanoparticles together and alsobetter adhesion of the coatings to glass substrates. See, e.g., U.S.patent application Ser. No. 11/268,574, filed Nov. 8, 2005, which isincorporated by reference in its entirety.

A similar calcination effect can be achieved at lower temperature (e.g.,less than 500° C., less than 250° C., less than 150° C., less than 125°C., or 100° C. or less) when the calcination is performed in thepresence of a suitable calcination reagent. The calcination reagent canpromote reaction between polyelectrolytes. For example the calcinationreagent can be selected to facilitate a hydrolysis reaction; water isone such calcination reagent. When the coating includes inorganicnanoparticles, the calcination reagent can promote reactions that formcovalent bonds between the nanoparticles.

The calcination conditions can be compatible with plastic materialswhich have low heat distortion temperatures (i.e., below 200° C.). Somesuch plastics include, for example, polyethylene terephthalate (PET),polycarbonate (PC), and polyimides. Hydrothermal calcination can includean exposure to steam at a temperature of 100° C. to 150° C. (e.g., 120°C.) at a pressure of 10 psi to 30 psi (e.g., 20 psi) for 0.5 hours to 8hours. The calcination can be carried out in an autoclave. Thehydrothermal calcination process can result in a coating that ishydrophilic but not superhydrophilic; as such the coating can lose itsanti-fogging properties upon calcination. The anti-fogging propertiescan be restored by a simple UV treatment, suggesting that the surfacewas becoming contaminated during calcination. Under certain calcinationconditions, the coating retains its superhydrophilicity.

The use of superheated steam, in particular for hydrothermal sinteringof silica gels, is known. Hydrothermal treatments have been appliedextensively to catalysts and sol-gel processes to obtain reaction mediasuitable for density-tunable structure syntheses and chemical surfacemodifications. See, for example, U.S. Pat. Nos. 2,739,075, 2,728,740,2,914,486, and 5,821,186, and EP 0 137 289, each of which isincorporated by reference in its entirety.

FIG. 1 shows coated article 10 including substrate 20 and coating 25 ona surface of substrate 20. Coating 25 includes nanoparticles 30 and 40.Nanoparticles 30 and 40 can have opposite electrostatic charges.Nanoparticles 30 and 40 can also have different compositions anddifferent average sizes. For example, nanoparticles 30 can besubstantially a titanium oxide, while nanoparticles 40 can besubstantially a silicon oxide. Nanoparticles 30 and 40 can be arrangedin coating 25 so as to create voids 50 among nanoparticles 30 and 40. Insome embodiments, coating 25 includes an organic polymer (e.g., apolyelectrolyte organic polymer such as PAA, PAH, or SPS). In otherembodiments, coating 25 is substantially free of an organic polymer.

Nanoparticle-based coatings, including coatings that are substantiallyfree of organic polymers, can be self-cleaning. An organic contaminantcan be removed or oxidized by the coating, e.g., upon exposure to anactivation light source. The activation light source can be a UV lightsource or a visible light source.

The coatings can be made by a layer-by-layer deposition process, inwhich a substrate is contacted sequentially with oppositely chargepolyelectrolytes. The polyelectrolytes can be in an aqueous solution.The substrate can be contacted with the aqueous solution by, forexample, immersion, printing, spin coating, spraying, mist deposition,or other methods.

The polyelectrolyte solutions can be applied in a single step, in whicha mixed polymer and nanoparticle solution is applied to a substrate in acontrolled manner to achieve required nano-porosity inside the coating.This approach can provide low fabrication cost and high yield.Alternatively, the polyelectrolyte solutions can be applied in amulti-step method, in which polymer layers and nano-particle layers aredeposited in an alternating fashion. The multi-step approach can be moreefficient for manufacturing with a spray method than an immersion-basedmethod, because spray deposition does not require a rinse betweenimmersions. With either method, the coating parameters such as materialcomposition, solution concentration, solvent type, and so on, can beoptimized to efficiently produce a coating with desired properties.

EXAMPLES

An all-nanoparticle multilayer of positively charged TiO₂ nanoparticles(average size ˜7 nm) and negatively charged SiO₂ nanoparticles (averagesize ˜7 and ˜22 nm) was prepared by layer-by-layer assembly using glassor silicon as the substrate. Each nanoparticle suspension had aconcentration of 0.03 wt. % and a pH of 3.0. The growth behavior ofmultilayers made of TiO₂ and SiO₂ nanoparticles was monitored usingspectroscopic ellipsometry and atomic force microscopy (AFM). FIG. 2Ashows the variation of film thickness with increasing number ofdeposited bilayers (one bilayer consists of a sequential pair of TiO₂and SiO₂ nanoparticle depositions). In both cases, the multilayers showlinear growth behavior (average bilayer thickness for 7 nm TiO₂/22 nmSiO₂ and 7 nm TiO₂/7 nm SiO₂ multilayers is 19.6 and 10.5 nm,respectively). The RMS surface roughness, determined via AFM, increasedasymptotically in each case. Other studies, in which nanoparticle thinfilms were assembled using polyelectrolytes, DNA or di-thiol compoundsas linkers between nanoparticles also showed linear growth behavior.See, for example, Ostrander, J. W., Mamedov, A. A., Kotov, N. A., J. Am.Chem. Soc. 2001, 123, 1101-1110; Lvov, Y.; Ariga, K.; Onda, M.;Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195-6203; Brust, M.;Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14,5425-5429; Taton, T. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.J. Am. Chem. Soc. 2000, 122, 6305-6306; and Cebeci, F. C.; Wu, Z. Z.;Zhai, L.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 2856-2862, eachof which is incorporated by reference in its entirety. However, in arecent molecular dynamics (MD) simulation study of layer-by-layerassembled multilayers comprising two oppositely charged nanoparticles,the thickness of the multilayers exhibited non-linear growth behaviordue to the increase in the surface roughness of the multilayers (Jeon,J.; Panchagnula, V.; Pan, J.; Dobrynin, A. V. Langmuir 2006, 22,4629-4637, which is incorporated by reference in its entirety). Theincreased surface roughness was a result of substrates not beinguniformly coated with the nanoparticles as the number of depositedlayers increased. Therefore, the observed saturation of surfaceroughness here indirectly indicates that the surface was uniformly andcompletely coated with the nanoparticle multilayers. AFM results (notshown) confirmed the existence of a uniform coating of multilayers onglass substrates.

While the refractive index of each system does not change as a functionof the number of deposited bilayers, the refractive indices of the twosystems differ. FIG. 2B shows that multilayers of 7 nm TiO₂ and 22 nmSiO₂ nanoparticles had an average refractive index of 1.28±0.01, whereasmultilayers made from 7 nm TiO₂ and 7 nm SiO₂ nanoparticles had anaverage refractive index of 1.32±0.01. In both cases, the refractiveindex of the TiO₂/SiO₂ nanoparticle multilayer was lower than thereported values of bulk anatase TiO₂ (2.0˜2.7) and SiO₂ (1.4˜1.5) (seeKlar, T.; et al., Phys. Rev. Lett. 1998, 80, 4249-4252; Wang, X. R.; etal., Appl. Phys. Lett. 1998, 72, 3264-3266; Biswas, R.; et al., Phys.Rev. B 2000, 61, 4549-4553; and Garcia-Santamaria, F.; et al., Langmuir2002, 18, 1942-1944, each of which is incorporated by reference in itsentirety). The assembly of nanoparticles results in the presence ofnanopores which effectively lower the refractive index of themultilayers. The invariance of the refractive index with thickness andthe linear growth behavior indicate that the composition of themultilayers for each system did not vary with increasing number ofdeposited bilayers. The difference in the observed refractive index ofthe two multilayer systems suggests that either the porosity, therelative amount of TiO₂ to SiO₂ nanoparticles, or both, differed. Toclearly address this issue, the porosity and chemical composition of thenanoparticle multilayer coatings was determined via ellipsometry.

Ellipsometry has been widely used to estimate the porosity of thin filmsbased on the assumption that the refractive index of the constituentmaterials is known. See, for example, Cebeci, F. C.; et al., Langmuir2006, 22, 2856-2862; and Tzitzinou, A.; et al., Macromolecules 2000, 33,2695-2708, each of which is incorporated by reference in its entirety.When the constituent materials are nanoparticles, however, it is notalways possible to have reliable information on the refractive index ofthe nanoparticles utilized to fabricate the film. The physicalproperties of nanoparticles differ from the bulk properties of theircorresponding materials due to quantum confinement effects and theirlarge specific surface areas (Henglein, A. Chem. Rev. 1989, 89,1861-1873; and Alivisatos, A. P. Science 1996, 271, 933-937, each ofwhich is incorporated by reference in its entirety).

A method based on ellipsometry to determine the porosity of nanoporousthin films without any assumption about the material properties (i.e.,refractive index) was developed. For porous thin films, if therefractive index of the material's framework (i.e., solid materials) andthe overall porosity are unknown, the solving of two independentequations containing the two parameters would be necessary to determinethese values. These two independent equations can be obtained bymeasuring the values of the effective refractive index of the porousthin films in two different media of known refractive index (e.g., airand water), assuming that the thickness of the porous thin films remainconstant in these two different media (constant volume). Anotherprerequisite for this method is that the pores should be interconnectedso that the chosen media can infiltrate and fill the pores completely.Based on the arguments above, the porosity and the refractive index ofthe film's solid framework can be expressed as follows:

$\begin{matrix}{p = {\frac{n_{f,2} - n_{f,1}}{n_{f,{water}} - n_{f,{air}}} = \frac{n_{f,2} - n_{f,1}}{0.33}}} & (1) \\{n_{f,{framwork}} = \frac{n_{f,1} - {n_{f,{air}} \cdot p}}{1 - p}} & (2)\end{matrix}$

where p represents the porosity (or the fraction of void volume) of theporous thin films, and n_(f,air) (1.00), n_(f,water) (=1.33), andn_(f,framework) represent the refractive index of air, water and thesolid framework, respectively. n_(f,1) and n_(f,2) represent theexperimentally measured effective refractive index of the porous thinfilms in media 1 (in air) and 2 (in water), respectively (the term“effective refractive index” (n_(f,1 and 2)) refers to the refractiveindex of the entire porous thin film experimentally measured viaellipsometry. On the other hand, the refractive index of the solidframework (N_(f,framework)) refers to the refractive index of the solidmaterials in the porous films). Equations (1) and (2) allowed thedetermination of the porosity of the thin film and the refractive indexof the nanoparticle framework with simple ellipsometric measurements intwo different media. As long as the solvent used (water was used in thisstudy) fills the pores but does not swell the structure, thismethodology can be used to characterize any nanoporous thin film,allowing facile determination of the porosity and refractive index offramework materials with an ellipsometer and a liquid cell. Also, bymaking thin films comprising only one type of nanoparticle, this methodallowed the determination of the refractive index of the constituentnanoparticle.

In the case of the all-nanoparticle thin films made from TiO₂ and SiO₂,four independent variables need to be determined for quantitativecharacterization of the films. These variables are porosity (p), thevolume fraction of either of the nanoparticles (e.g., v_(TiO) ₂ ), andthe refractive indices of TiO₂ (n_(f,TiO) ₂ ) and SiO₂ (n_(f,SiO) ₂ )nanoparticles. The refractive index of each nanoparticle (n_(f,TiO) ₂and n_(f,SiO) ₂ ) was first obtained by the method described above; thatis, effective refractive indices of nanoporous thin films comprisingeither TiO₂ or SiO₂ nanoparticles were measured in air and in water, andthen equations (1) and (2) were used to calculate the refractive indexof each constituent nanoparticle. For this purpose, nanoporous thinfilms comprising either all-TiO₂ or all-SiO₂ nanoparticles were preparedby the layer-by-layer assembly of TiO₂ nanoparticle/poly(vinylsulfonate) (PVS) or poly(diallyldimethylammonium chloride) (PDAC)/SiO₂nanoparticle multilayers, respectively. To prevent swelling of thesemultilayers, the polymers in each multilayer were subsequently removedand the constituent nanoparticles were partially fused together by hightemperature calcination before ellipsometric measurements were performedin air and in water. The calcinated films did not undergo any swellingin water. The refractive indices of 7 nm TiO₂, 7 nm SiO₂ and 22 nm SiO₂nanoparticles were determined to be 2.21±0.05, 1.47±0.01 and 1.47±0.004,respectively. The porosity (p) and the refractive index of the compositeframework (n_(f,framework); the term “composite” is used as thematerial's framework in this case since it consists of TiO₂ and SiO₂nanoparticles) of the TiO₂/SiO₂ nanoparticle-based films were determinedby measuring the effective refractive index of these TiO₂/SiO₂nanoparticle-based films in air and in water, and using equations (1)and (2). These all-nanoparticle thin films were also calcinated (as willbe described below) to prevent swelling of the films in water. Using thevalues obtained for n_(f,TiO) ₂ , n_(f,SiO) ₂ , n_(f,framework) and p,the volume fraction of TiO₂ and SiO₂ nanoparticles can be calculatedusing the linear relation for composite refractive indices and isexpressed as shown in the following equations:

$\begin{matrix}{v_{{TiO}_{2}} = {\frac{n_{f,{framework}} - n_{f,{SiO}_{2}}}{n_{f,{TiO}_{2}} - n_{f,{SiO}_{2}}} \cdot \left( {1 - p} \right)}} & (3) \\{v_{{SiO}_{2}} = {1 - \left( {p + v_{{TiO}_{2}}} \right)}} & (4)\end{matrix}$

The values obtained are summarized in Table 1. The major differencebetween the 7 nm TiO₂/22 nm SiO₂ and 7 nm TiO₂/7 nm SiO₂nanoparticle-based multilayer coatings was the porosity, consistent witha denser packing of nanoparticles in films with 7 nm TiO₂ and 7 nm SiO₂nanoparticles compared to films with the 22 nm SiO₂ nanoparticles. Theweight fraction of TiO₂ nanoparticles in the two systems did not differsignificantly, although the 7 nm TiO₂/7 nm SiO₂ system had a slightlylarger value. Table 1 also shows that the ellipsometry method wassensitive enough to distinguish the slight difference in chemicalcomposition of multilayers with a half bilayer difference (e.g., between6 and 6.5 bilayers of 7 nm TiO₂ and 22 nm SiO₂ multilayers).

TABLE 1 Porosity and chemical composition of calcinated TiO₂/SiO₂multilayers as determined by in-situ ellipsometric method. CompositionNumber of vol. % (wt. %) Multilayers bilayers Air TiO₂ ^(a) SiO₂ ^(b) (7nm TiO₂/22 nm SiO₂) 6 44.7 (0) 1.2 (6.3) 54.1 (93.7) 6.5 45.3 (0) 1.6(8.0) 53.1 (92.0) (7 nm TiO₂/7 nm SiO₂) 12 35.4 (0) 1.6 (7.3) 63.0(92.7) 12.5 35.8 (0) 1.7 (8.0) 62.5 (92.0) ^(a)density of TiO₂ = 3.9g/cm³, ^(b)density of 22 nm and 7 nm SiO₂ = 1.3 and 1.22 g/cm³ (providedby the supplier), respectively.

To confirm the reliability of the chemical composition determined viaellipsometry, the weight fractions of TiO₂ and SiO₂ nanoparticles weredetermined independently using a quartz crystal microbalance (QCM) andX-ray photoelectron spectroscopy (XPS). Table 2 summarizes the chemicalcomposition (wt. % of TiO₂ nanoparticles) determined via QCM and XPS.The weight fractions of TiO₂ obtained from QCM and XPS consistentlyindicated that the amount of TiO₂ nanoparticles in the multilayers wasrelatively small (<12 wt. %) and that the 7 nm TiO₂/7 nm SiO₂multilayers had a slightly larger amount of TiO₂ nanoparticles presentin the films. These results were consistent with the results obtainedfrom ellipsometry which showed that the weight fraction of TiO₂nanoparticles in both systems was relatively small compared to that ofthe SiO₂ nanoparticles and that the 7 nm TiO₂/7 nm SiO₂ system had ahigher content of TiO₂ nanoparticles. The fact that the values obtainedfrom three different techniques showed good agreement validated thecapability of the ellipsometry method.

TABLE 2 Weight percentage (wt. %) of TiO₂ nanoparticles in TiO₂/SiO₂nanoparticle thin films as determined by QCM and XPS. Multilayers QCMXPS^(a) (7 nm TiO₂/22 nm SiO₂)  8.1 ± 2.3 2.9~6.6  (7 nm TiO₂/7 nm SiO₂)11.6 ± 1.7 5.8~10.9 ^(a)The lower and upper limit of the values for TiO₂wt. % were obtained by analyzing the multilayers with SiO₂ and TiO₂nanoparticles as the outermost layer, respectively.

The observation that the volume fraction of TiO₂ nanoparticles in thetwo multilayer systems studied was below 2 vol. % (less than 8 wt. %)was remarkable and surprising. The surface charge density of eachnanoparticle during the LbL assembly can play an important role indetermining the chemical composition of the TiO₂/SiO₂ nanoparticle-basedmultilayer thin films. At the assembly conditions, which was pH 3.0 forboth nanoparticle suspensions, the zeta-potential of the 7 nm TiO₂nanoparticles was +40.9±0.9 mV, compared to values of −3.3±2.6 and−13.4±1.4 mV, for the 7 nm and 22 nm SiO₂ nanoparticles respectively.These values suggest that the TiO₂ nanoparticles were much more highlycharged than the SiO₂ nanoparticles during the LbL assembly. Therefore,only a small number of TiO₂ nanoparticles would be required to achievethe charge reversal required for multilayer growth. Also theinterparticle distance between adsorbed TiO₂ nanoparticles would belarge due to strong electrostatic repulsion between the particles. Onthe other hand, a large number of SiO₂ nanoparticles would be needed toreverse the surface charge and, at the same time, SiO₂ nanoparticles canpack more densely compared to TiO₂ nanoparticles as the electrostaticrepulsion between the SiO₂ nanoparticles was not as great as thatbetween highly charged TiO₂ nanoparticles. On a similar note, Lvov etal. have also shown that the partial neutralization of functional groupson SiO₂ nanoparticles by addition of salt to a nanoparticle suspensionleads to an increased fraction of SiO₂ nanoparticles in multilayersassembled with a polycation (Lvov, Y.; et al., Langmuir 1997, 13,6195-6203, which is incorporated by reference in its entirety).

FIGS. 3A-3D show the effects of pH on nanoparticle film assembly. FIG.3A shows average bilayer thickness as a function of nanoparticlesolution pH (both solutions were at the same pH). FIG. 3B shows themeasured zeta-potential of 7 nm TiO₂ and 22 nm SiO₂ nanoparticles as afunction of pH. FIG. 3C shows the measured particle sizes as a functionof pH. FIG. 3D shows the effect of pH of each nanoparticle solution onaverage bilayer thickness. These data can be used to select assemblyconditions to control coating properties (e.g., final coatingthickness).

Depositing the nanoporous TiO₂/SiO₂ nanoparticle-based coatings on glasscaused the reflective losses in the visible region to be significantlyreduced, and transmission levels above 99% were be readily achieved. Thewavelength of maximum suppression of reflections in the visible regionwas determined by the quarter-wave optical thickness of the coatings,which can be varied by changing the number of layers deposited as seenin FIG. 4A. FIG. 4A shows transmittance spectra of 7 nm TiO₂/22 nm SiO₂multilayer coatings before (thin solid line) and after calcination at550° C. (thick solid line) on glass substrates. Green, Red and Bluecurves represent transmittance through untreated glass and glass coatedwith 5- and 6 bilayers, respectively. FIG. 4B reveals the visual impactof these all-nanoparticle antireflection coatings. FIG. 4B is aphotograph of a glass slide showing the suppression of reflection by a 5bilayer 7 nm TiO₂/22 nm SiO₂ nanoparticle multilayer (calcinated). Theleft portion of the slide was not coated with the multilayers.Multilayer coatings are on both sides of the glass substrates. Due toits higher effective refractive index, the antireflection properties ofa multilayer coating made from 7 nm TiO₂ and 7 nm SiO₂ nanoparticles(not shown) were not as pronounced (ca. 98 and 97% maximum transmissionin the visible region before and after calcination, respectively) as the7 nm TiO₂ and 22 nm SiO₂ nanoparticle-based multilayer coatings. Thewavelength of maximum suppression of the 7 nm TiO₂/7 nm SiO₂nanoparticle system, however, can be tuned more precisely compared tothe 7 nm TiO₂/22 nm SiO₂ multilayers as the average bilayer thickness isonly 10 nm.

For practical application of any coating, the mechanical integrity(durability and adhesion) can be extremely important. As-assembledTiO₂/SiO₂ nanoparticle-based multilayers show less than ideal mechanicalproperties. The poor adhesion and durability of the as-assembledmultilayer films was likely due to the absence of any interpenetratingcomponents (i.e., charged macromolecules) that bridge or glue thedeposited particles together within the multilayers. The mechanicalproperties of the all-nanoparticle multilayers were improvedsignificantly by calcinating the as-assembled multilayers at a hightemperature (550° C.) for 3 h. As described briefly above, this processled to the partial fusing of the nanoparticles together. Better adhesionof the coatings to glass substrates was also achieved (see Cebeci, F.C.; et al., Langmuir 2006, 22, 2856-2862, which is incorporated byreference in its entirety). While the film thickness decreased by ca.5%, the refractive index increased slightly (ca. 2%) after thecalcination process, resulting in the observed blue shift in the maximumtransmission wavelength as seen in FIG. 4A. From this point on, themultifunctional properties of calcinated (7 nm TiO₂/22 nm SiO₂)multilayers will be reported.

In addition to antireflection properties, the nanoporosity of TiO₂/SiO₂nanoparticle multilayers led to superhydrophilicity. Nanoporous coatingsinclude SiO₂ nanoparticles exhibit superhydrophilicity (water dropletcontact angle <5° in less than 0.5 sec) due to the nanowicking of waterinto the network of capillaries present in the coatings (see U.S. patentapplication Ser. No. 11/268,547, and Cebeci, F. C.; et al., Langmuir2006, 22, 2856-2862, each of which is incorporated by reference in itsentirety). The mechanism of such behavior can be understood from thesimple relation derived by Wenzel and co-workers. It is well establishedthat the apparent contact angle of a liquid on a surface depends on theroughness of the surface according to the following relation:

cos θ_(a) =r cos θ  (5)

where θ_(a) is the apparent water contact angle on a rough surface and θis the intrinsic contact angle as measured on a smooth surface. r is thesurface roughness defined as the ratio of the actual surface area overthe project surface area. r becomes infinite for porous materialsmeaning that the surface will be completely wetted (i.e., θ_(a)˜θ) withany liquid that has a contact angle (as measured on a smooth surface) ofless than 90°. The contact angle of water on a planar SiO₂ and TiO₂surface is reported to be approximately 20° and 50˜70°, respectively;therefore, multilayers comprised of SiO₂ nanoparticles (majoritycomponent) and TiO₂ nanoparticles (minority component) with nanoporousstructures should exhibit superhydrophilicity. FIGS. 5A-B verified thisexpectation; the data show that the contact angle of a water droplet(˜0.5 μL) on TiO₂/SiO₂ nanoparticle-based multilayer coatings becameless than 5° in less than 0.5 sec.

Unlike TiO₂-based coatings which lose their superhydrophilicity in thedark, SiO₂/TiO₂ nanoparticle-based coatings retained thesuperhydrophilicity even after being stored in dark for months at atime. This can be because the superhydrophilicity is enabled by thenanoporous structure rather than the chemistry of TiO₂. FIG. 5B showsthat superhydrophilicity remained after 60 days of storage in the dark.

The anti-fogging properties of these coatings were demonstrated byexposing untreated and multilayer-modified glass substrates to humidenvironments (relative humidity ˜50%) after cooling them at a lowtemperature (4° C.) as seen in FIG. 5C. FIG. 5C is a photographdemonstrating the anti-fogging properties of multilayer coated glass(left) compared to that of an untreated glass substrate (right). Eachsample was exposed to air (relative humidity ˜50%) after being cooled ina refrigerator (4° C.) for 12 h. The top portion of the slide on theleft had not been coated with the multilayer. Although ordinary (i.e.,solid) TiO₂ based coatings can be rendered superhydrophilic andanti-fogging by UV irradiation, such coatings lose their anti-foggingproperties after storage in dark. The TiO₂/SiO₂ nanoparticle basedmultilayers retained their anti-fogging properties even after beingstored in dark over 6 months. As described above, the presence ofnanopores in these films leads to nanowicking of water into the networkof capillaries in the coatings; therefore, the superhydrophilicity ofthese coatings can be observed even in the absence of UV irradiation.

Contamination of the porous matrix by organic compounds can lead to theloss of anti-fogging properties. In this respect, a self-cleaning ananti-fogging coating can be desirable, to promote long-term performanceof the anti-fogging coating. The self-cleaning properties of TiO₂/SiO₂nanoparticle-based multilayers was tested to confirm that organiccontaminants can be removed or oxidized under UV irradiation. Glasssubstrates coated with TiO₂/SiO₂ nanoparticle-based multilayers and SiO₂nanoparticle-based superhydrophilic coatings were contaminated using amodel contaminant, i.e., methylene blue (MB). The decomposition ofmethylene blue by the coatings was monitored by measuring the amount ofremaining MB in the coatings after UV irradiation as a function of time.FIG. 6 shows that essentially more than 90% of the MB in the TiO₂/SiO₂nanoparticle-based coatings (diamonds) was decomposed after 3 h of UVirradiation. In a coating with only SiO₂ nanoparticles (squares), morethan 60% of the MB remained in the coating after 4 h. The contact angleof water on the MB-contaminated surface was 20.0±1.2° and changed to ˜3°after 2 h of UV irradiation indicating that the superhydrophilicity wasalso recovered. These results demonstrate that the amount of TiO₂nanoparticles in the multilayer coatings (slightly more than 1 volume %)was sufficient to confer self-cleaning properties to these coatings. Therecovered anti-fogging property was retained for more than 30 days evenafter storing the UV illuminated samples in dark.

The contact angle of water on the MB contaminated TiO₂/SiO₂ surface was18.5±1.00; thus, the antifogging properties were lost. The contact anglechanged to ˜0° after 2 h of UV irradiation indicating that thesuperhydrophilicity was recovered. These results demonstrated that theamount of TiO₂ nanoparticles in the multilayer coatings (slightly morethan 6 wt. %) was sufficient to confer self-cleaning properties to thesecoatings. Nakajima et al. also have shown that transparentsuperhydrophobic coatings containing only 2 wt. % TiO₂ can self-cleanunder the action of UV irradiation (Nakajima, A.; et al., Langmuir 2000,16, 7044-7047, which is incorporated by reference in its entirety). Therecovered antifogging property was retained for more than 30 days evenafter storing the MB-contaminated/UV illuminated samples in the dark.The contact angle measured on the UV irradiated samples after 30 days ofstorage in the dark was less than 40, which is below the limit at whichantifogging properties are observed (˜70). Some previous studies haveshown that the incorporation of SiO₂ into TiO₂ based thin films improvesthe stability of light-induced superhydrophilicity; however, themechanism behind the improved stability was different from ourmechanism, which is driven by the nanoporosity (see, e.g., Zhang, X. T.;et al., Chem. Mater. 2005, 17, 696-700; and Machida, M., et al., J.Mater. Sci. 1999, 34, 2569-2574, each of which is incorporated byreference in its entirety).

While the current system contains TiO₂ nanoparticles that require UVirradiation (wavelengths shorter than 400 nm) for activation, TiO₂nanoparticles that are sensitive to visible light have been developed.We believe incorporation of these visible light active TiO₂nanoparticles into the multilayer coatings should be straightforwardusing the LbL technique and enable self-cleaning properties of thecoatings in the visible region.

FIGS. 7A-B show atomic force microscopy (AFM) images of pre- (FIG. 7A)and post-treatment (FIG. 7B) silica surfaces. There was extensivebridging between nanoparticles post-treatment.

FIG. 8 shows reflection spectra of an uncoated glass slide and ahydrothermally stabilized silica-based anti-reflection coating on glasssubstrate. The uncoated slide reflected approximately 8% of incidentlight across the visible spectrum; the coated, treated slide had a broadreflectivity window centered at approximately 580 nm, with a minimumreflectance value of less than 2%.

The effect of wiping the anti-reflection coating above with 70%isopropanol is demonstrated in FIGS. 9A-B. The wavelengths at whichanti-reflection coatings were functional depended strongly on thethickness of the coatings. Therefore, superimposed transmission curvesbefore and after wiping with isopropanol demonstrate that the coatingwas not being removed from the substrate upon wiping. Anti-reflectioncoatings as-prepared (i.e., not calcinated) were removed immediatelyupon wiping (not shown).

FIG. 10 shows the results of nanoindentation experiments on as-prepared,thermally calcined (550° C.), and hydrothermally treated coatings. Theas-prepared film had a nanoindentation modulus of less than 10 GPa, thethermally calcinated film had a nanoindentation modulus of less than 15GPa, and the hydrothermally treated (i.e., exposed to pressurized steamat 120° C.) film had a nanoindentation modulus of greater than 15 GPa.

In addition to providing mechanical stability to films, hydrothermaltreatments were used to generate surface patterns in a completelyself-assembled, bottom-up manner. While such pattern generationnecessitates careful selection of coating composition, an example isshown in FIGS. 11A-B, which are atomic force micrographs ofhydrothermally treated coatings. Such pattern generation techniques mayprove important in mimicking biological pattern formation duringbiological development (e.g., butterfly wings) in a cheap, reliable, andscalable fashion.

Other embodiments are within the scope of the following claims.

1. A method of treating a surface comprising: depositing a firstplurality of inorganic nanoparticles having a first electrostatic chargeon a substrate; depositing an oppositely charged polyelectrolyte overthe first plurality of inorganic nanoparticles; and contacting the firstplurality of inorganic nanoparticles and the oppositely chargedpolyelectrolyte with a calcination reagent at a calcination temperature.2. The method of claim 1, wherein the oppositely charged polyelectrolyteincludes a second plurality of inorganic nanoparticles.
 3. The method ofclaim 2, wherein the first plurality of inorganic nanoparticles has adifferent average particle size than the second plurality of inorganicnanoparticles.
 4. The method of claim 2, wherein the first plurality ofinorganic nanoparticles includes a plurality of silicon dioxidenanoparticles.
 5. The method of claim 4, wherein the second plurality ofinorganic nanoparticles includes a plurality of titanium dioxidenanoparticles.
 6. The method of claim 1, wherein the calcinationtemperature is less than 500° C.
 7. The method of claim 1, wherein thecalcination temperature is less than 200° C.
 8. The method of claim 1,wherein the calcination temperature is less than 150° C.
 9. The methodof claim 8, wherein the calcination reagent is water.
 10. The method ofclaim 8, contacting the first plurality of inorganic nanoparticles andthe oppositely charged polyelectrolyte with a calcination reagent at acalcination temperature includes contacting at a calcination pressure.11. The method of claim 11, wherein the calcination pressure is in therange of 10 psi to 30 psi.
 12. The method of claim 1, further comprisingrepeating the steps of depositing a first plurality of inorganicnanoparticles having a first electrostatic charge on a substrate anddepositing an oppositely charged polyelectrolyte over the firstplurality of inorganic nanoparticles; thereby forming an electrostaticmultilayer.
 13. The method of claim 12, wherein the electrostaticmultilayer is substantially free of an organic polymer.
 14. An articlecomprising a surface treated by the method of claim
 1. 15. Asuperhydrophilic surface comprising a plurality of hydrothermallycalcinated inorganic nanoparticles arranged on a substrate.
 16. Thesurface of claim 15, wherein the surface has a nanoindentation modulusof greater than 15 GPa.